We propose to study the basis for diagnostic decision-making strategies in several common groups of "tracer" illnesses, possibly including: respiratory tract infections; urinary and vaginal infections (women); acute onset nausea, vomiting, diarrhea, and/or abdominal pain; chest pain; trauma to extremities. We will conduct prospective studies at two Health Maintenance Organizations designed to correlate a single clinical finding (history or physical examination) or a complex of clinical findings with the presence or absence of a given final diagnosis. Further, given a particular complex of findings we will measure the incremental gain (or loss) of a specific costly procedure--typically, a laboratory test--in ruling in or ruling out a particular diagnosis. As a function of this analysis, we will attempt to estimate the specific clinical features which enhance or detract from the "yield" of each laboratory test. We are proposing to collect information which is essential in designing cost-effective strategies for diagnosing conditions which are manifested by common symptoms or syndromes. But while such information is essential, it is not sufficient, for the development of a cost-effective strategy requires definitions of both "cost" and "effectiveness". And those definitions require value judgements which may differ from one individual or peer group to another (e.g., the "cost" of suffering for several days because of delay in treatment) or additional information which is currently unavailable (e.g., reduction in days of functional impairment resulting from treatment).